1. Field of the Invention
The invention relates to a recording system, and, in particular, to one in which information is recorded with a laser in a radiation absorbing film.
2. Description of the Prior Art
Improvements in apparatus for recording information have been described by D. Maydan, M. I. Cohen, and R. E. Kerwin in U.S. Pat. No. 3,720,784, issued Mar. 13, 1973. In that patent is described apparatus capable of forming a large number of short duration amplitude-modulated pulses of spatially coherent radiation to create positive or negative pictorial images. The images consist of a pattern of small discrete holes in a thin radiation absorbing film. In one typical mode of operation, the short laser pulses evaporate a small amount of the film in the center of the spot upon which the beam is incident and melt a large area around this region. Surface tension then draws the melted material toward the rim of the melted area, thereby displacing the film from a nearly circular region of the transparent substrate. By varying the amplitude of the very short laser pulses, the diameter of the region that is melted can be varied, and the area of the increasing hole increases monotonically with increasing pulse amplitude. The holes are formed in parallel rows with the centers of the holes equally spaced along each row and from row to row. The largest holes are of diameter nearly equal to the center-to-center spacing of the holes. In this way, it is possible to achieve a wide range of shades of grey. The apparatus is particularly useful for recording graphic copy or images that are transmitted over telephone lines, such as from facsimile transmitters.
In that patent, the preferred radiation absorbing film comprises a thin layer of bismuth (e.g., about 500 Angstroms) deposited on a polyester substrate such as Mylar (trademark of E. I. Dupont de Nemours and Co., Inc.). In U.S. Pat. No. 3,560,994, issued Feb. 2, 1971 to K. Wolff and H. Hamisch, it is taught that the properties of bismuth films are improved by superimposing a coating which decreases the reflectivity of the incident laser beam. Specifically, that patent teaches that such an anti-reflection film must have an index of refraction n of about 4, and, accordingly, silicon (n = 4.5) or germanium (n = 4.4) are preferred.